/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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 */

package java.security;

import sun.security.util.Debug;
import sun.reflect.CallerSensitive;
import sun.reflect.Reflection;

/**
 * <p> The AccessController class is used for access control operations
 * and decisions.
 *
 * <p> More specifically, the AccessController class is used for
 * three purposes:
 *
 * <ul>
 * <li> to decide whether an access to a critical system
 * resource is to be allowed or denied, based on the security policy
 * currently in effect,
 * <li>to mark code as being "privileged", thus affecting subsequent
 * access determinations, and
 * <li>to obtain a "snapshot" of the current calling context so
 * access-control decisions from a different context can be made with
 * respect to the saved context. </ul>
 *
 * <p> The {@link #checkPermission(Permission) checkPermission} method
 * determines whether the access request indicated by a specified
 * permission should be granted or denied. A sample call appears
 * below. In this example, {@code checkPermission} will determine
 * whether or not to grant "read" access to the file named "testFile" in
 * the "/temp" directory.
 *
 * <pre>
 *
 * FilePermission perm = new FilePermission("/temp/testFile", "read");
 * AccessController.checkPermission(perm);
 *
 * </pre>
 *
 * <p> If a requested access is allowed,
 * {@code checkPermission} returns quietly. If denied, an
 * AccessControlException is
 * thrown. AccessControlException can also be thrown if the requested
 * permission is of an incorrect type or contains an invalid value.
 * Such information is given whenever possible.
 *
 * Suppose the current thread traversed m callers, in the order of caller 1
 * to caller 2 to caller m. Then caller m invoked the
 * {@code checkPermission} method.
 * The {@code checkPermission} method determines whether access
 * is granted or denied based on the following algorithm:
 *
 * <pre> {@code
 * for (int i = m; i > 0; i--) {
 *
 *     if (caller i's domain does not have the permission)
 *         throw AccessControlException
 *
 *     else if (caller i is marked as privileged) {
 *         if (a context was specified in the call to doPrivileged)
 *             context.checkPermission(permission)
 *         if (limited permissions were specified in the call to doPrivileged) {
 *             for (each limited permission) {
 *                 if (the limited permission implies the requested permission)
 *                     return;
 *             }
 *         } else
 *             return;
 *     }
 * }
 *
 * // Next, check the context inherited when the thread was created.
 * // Whenever a new thread is created, the AccessControlContext at
 * // that time is stored and associated with the new thread, as the
 * // "inherited" context.
 *
 * inheritedContext.checkPermission(permission);
 * }</pre>
 *
 * <p> A caller can be marked as being "privileged"
 * (see {@link #doPrivileged(PrivilegedAction) doPrivileged} and below).
 * When making access control decisions, the {@code checkPermission}
 * method stops checking if it reaches a caller that
 * was marked as "privileged" via a {@code doPrivileged}
 * call without a context argument (see below for information about a
 * context argument). If that caller's domain has the
 * specified permission and at least one limiting permission argument (if any)
 * implies the requested permission, no further checking is done and
 * {@code checkPermission}
 * returns quietly, indicating that the requested access is allowed.
 * If that domain does not have the specified permission, an exception
 * is thrown, as usual. If the caller's domain had the specified permission
 * but it was not implied by any limiting permission arguments given in the call
 * to {@code doPrivileged} then the permission checking continues
 * until there are no more callers or another {@code doPrivileged}
 * call matches the requested permission and returns normally.
 *
 * <p> The normal use of the "privileged" feature is as follows. If you
 * don't need to return a value from within the "privileged" block, do
 * the following:
 *
 * <pre> {@code
 * somemethod() {
 *     ...normal code here...
 *     AccessController.doPrivileged(new PrivilegedAction<Void>() {
 *         public Void run() {
 *             // privileged code goes here, for example:
 *             System.loadLibrary("awt");
 *             return null; // nothing to return
 *         }
 *     });
 *     ...normal code here...
 * }}</pre>
 *
 * <p>
 * PrivilegedAction is an interface with a single method, named
 * {@code run}.
 * The above example shows creation of an implementation
 * of that interface; a concrete implementation of the
 * {@code run} method is supplied.
 * When the call to {@code doPrivileged} is made, an
 * instance of the PrivilegedAction implementation is passed
 * to it. The {@code doPrivileged} method calls the
 * {@code run} method from the PrivilegedAction
 * implementation after enabling privileges, and returns the
 * {@code run} method's return value as the
 * {@code doPrivileged} return value (which is
 * ignored in this example).
 *
 * <p> If you need to return a value, you can do something like the following:
 *
 * <pre> {@code
 * somemethod() {
 *     ...normal code here...
 *     String user = AccessController.doPrivileged(
 *         new PrivilegedAction<String>() {
 *         public String run() {
 *             return System.getProperty("user.name");
 *             }
 *         });
 *     ...normal code here...
 * }}</pre>
 *
 * <p>If the action performed in your {@code run} method could
 * throw a "checked" exception (those listed in the {@code throws} clause
 * of a method), then you need to use the
 * {@code PrivilegedExceptionAction} interface instead of the
 * {@code PrivilegedAction} interface:
 *
 * <pre> {@code
 * somemethod() throws FileNotFoundException {
 *     ...normal code here...
 *     try {
 *         FileInputStream fis = AccessController.doPrivileged(
 *         new PrivilegedExceptionAction<FileInputStream>() {
 *             public FileInputStream run() throws FileNotFoundException {
 *                 return new FileInputStream("someFile");
 *             }
 *         });
 *     } catch (PrivilegedActionException e) {
 *         // e.getException() should be an instance of FileNotFoundException,
 *         // as only "checked" exceptions will be "wrapped" in a
 *         // PrivilegedActionException.
 *         throw (FileNotFoundException) e.getException();
 *     }
 *     ...normal code here...
 *  }}</pre>
 *
 * <p> Be *very* careful in your use of the "privileged" construct, and
 * always remember to make the privileged code section as small as possible.
 * You can pass {@code Permission} arguments to further limit the
 * scope of the "privilege" (see below).
 *
 *
 * <p> Note that {@code checkPermission} always performs security checks
 * within the context of the currently executing thread.
 * Sometimes a security check that should be made within a given context
 * will actually need to be done from within a
 * <i>different</i> context (for example, from within a worker thread).
 * The {@link #getContext() getContext} method and
 * AccessControlContext class are provided
 * for this situation. The {@code getContext} method takes a "snapshot"
 * of the current calling context, and places
 * it in an AccessControlContext object, which it returns. A sample call is
 * the following:
 *
 * <pre>
 *
 * AccessControlContext acc = AccessController.getContext()
 *
 * </pre>
 *
 * <p>
 * AccessControlContext itself has a {@code checkPermission} method
 * that makes access decisions based on the context <i>it</i> encapsulates,
 * rather than that of the current execution thread.
 * Code within a different context can thus call that method on the
 * previously-saved AccessControlContext object. A sample call is the
 * following:
 *
 * <pre>
 *
 * acc.checkPermission(permission)
 *
 * </pre>
 *
 * <p> There are also times where you don't know a priori which permissions
 * to check the context against. In these cases you can use the
 * doPrivileged method that takes a context. You can also limit the scope
 * of the privileged code by passing additional {@code Permission}
 * parameters.
 *
 * <pre> {@code
 * somemethod() {
 *     AccessController.doPrivileged(new PrivilegedAction<Object>() {
 *         public Object run() {
 *             // Code goes here. Any permission checks within this
 *             // run method will require that the intersection of the
 *             // caller's protection domain and the snapshot's
 *             // context have the desired permission. If a requested
 *             // permission is not implied by the limiting FilePermission
 *             // argument then checking of the thread continues beyond the
 *             // caller of doPrivileged.
 *         }
 *     }, acc, new FilePermission("/temp/*", read));
 *     ...normal code here...
 * }}</pre>
 * <p> Passing a limiting {@code Permission} argument of an instance of
 * {@code AllPermission} is equivalent to calling the equivalent
 * {@code doPrivileged} method without limiting {@code Permission}
 * arguments. Passing a zero length array of {@code Permission} disables
 * the code privileges so that checking always continues beyond the caller of
 * that {@code doPrivileged} method.
 *
 * @author Li Gong
 * @author Roland Schemers
 * @see AccessControlContext
 */

public final class AccessController {

  /**
   * Don't allow anyone to instantiate an AccessController
   */
  private AccessController() {
  }

  /**
   * Performs the specified {@code PrivilegedAction} with privileges
   * enabled. The action is performed with <i>all</i> of the permissions
   * possessed by the caller's protection domain.
   *
   * <p> If the action's {@code run} method throws an (unchecked)
   * exception, it will propagate through this method.
   *
   * <p> Note that any DomainCombiner associated with the current
   * AccessControlContext will be ignored while the action is performed.
   *
   * @param <T> the type of the value returned by the PrivilegedAction's {@code run} method.
   * @param action the action to be performed.
   * @return the value returned by the action's {@code run} method.
   * @throws NullPointerException if the action is {@code null}
   * @see #doPrivileged(PrivilegedAction, AccessControlContext)
   * @see #doPrivileged(PrivilegedExceptionAction)
   * @see #doPrivilegedWithCombiner(PrivilegedAction)
   * @see java.security.DomainCombiner
   */

  @CallerSensitive
  public static native <T> T doPrivileged(PrivilegedAction<T> action);

  /**
   * Performs the specified {@code PrivilegedAction} with privileges
   * enabled. The action is performed with <i>all</i> of the permissions
   * possessed by the caller's protection domain.
   *
   * <p> If the action's {@code run} method throws an (unchecked)
   * exception, it will propagate through this method.
   *
   * <p> This method preserves the current AccessControlContext's
   * DomainCombiner (which may be null) while the action is performed.
   *
   * @param <T> the type of the value returned by the PrivilegedAction's {@code run} method.
   * @param action the action to be performed.
   * @return the value returned by the action's {@code run} method.
   * @throws NullPointerException if the action is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see java.security.DomainCombiner
   * @since 1.6
   */
  @CallerSensitive
  public static <T> T doPrivilegedWithCombiner(PrivilegedAction<T> action) {
    AccessControlContext acc = getStackAccessControlContext();
    if (acc == null) {
      return AccessController.doPrivileged(action);
    }
    DomainCombiner dc = acc.getAssignedCombiner();
    return AccessController.doPrivileged(action,
        preserveCombiner(dc, Reflection.getCallerClass()));
  }


  /**
   * Performs the specified {@code PrivilegedAction} with privileges
   * enabled and restricted by the specified {@code AccessControlContext}.
   * The action is performed with the intersection of the permissions
   * possessed by the caller's protection domain, and those possessed
   * by the domains represented by the specified {@code AccessControlContext}.
   * <p>
   * If the action's {@code run} method throws an (unchecked) exception,
   * it will propagate through this method.
   * <p>
   * If a security manager is installed and the specified
   * {@code AccessControlContext} was not created by system code and the
   * caller's {@code ProtectionDomain} has not been granted the
   * {@literal "createAccessControlContext"}
   * {@link java.security.SecurityPermission}, then the action is performed
   * with no permissions.
   *
   * @param <T> the type of the value returned by the PrivilegedAction's {@code run} method.
   * @param action the action to be performed.
   * @param context an <i>access control context</i> representing the restriction to be applied to
   * the caller's domain's privileges before performing the specified action.  If the context is
   * {@code null}, then no additional restriction is applied.
   * @return the value returned by the action's {@code run} method.
   * @throws NullPointerException if the action is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedExceptionAction, AccessControlContext)
   */
  @CallerSensitive
  public static native <T> T doPrivileged(PrivilegedAction<T> action,
      AccessControlContext context);


  /**
   * Performs the specified {@code PrivilegedAction} with privileges
   * enabled and restricted by the specified
   * {@code AccessControlContext} and with a privilege scope limited
   * by specified {@code Permission} arguments.
   *
   * The action is performed with the intersection of the permissions
   * possessed by the caller's protection domain, and those possessed
   * by the domains represented by the specified
   * {@code AccessControlContext}.
   * <p>
   * If the action's {@code run} method throws an (unchecked) exception,
   * it will propagate through this method.
   * <p>
   * If a security manager is installed and the specified
   * {@code AccessControlContext} was not created by system code and the
   * caller's {@code ProtectionDomain} has not been granted the
   * {@literal "createAccessControlContext"}
   * {@link java.security.SecurityPermission}, then the action is performed
   * with no permissions.
   *
   * @param <T> the type of the value returned by the PrivilegedAction's {@code run} method.
   * @param action the action to be performed.
   * @param context an <i>access control context</i> representing the restriction to be applied to
   * the caller's domain's privileges before performing the specified action.  If the context is
   * {@code null}, then no additional restriction is applied.
   * @param perms the {@code Permission} arguments which limit the scope of the caller's privileges.
   * The number of arguments is variable.
   * @return the value returned by the action's {@code run} method.
   * @throws NullPointerException if action or perms or any element of perms is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedExceptionAction, AccessControlContext)
   * @since 1.8
   */
  @CallerSensitive
  public static <T> T doPrivileged(PrivilegedAction<T> action,
      AccessControlContext context, Permission... perms) {

    AccessControlContext parent = getContext();
    if (perms == null) {
      throw new NullPointerException("null permissions parameter");
    }
    Class<?> caller = Reflection.getCallerClass();
    return AccessController.doPrivileged(action, createWrapper(null,
        caller, parent, context, perms));
  }


  /**
   * Performs the specified {@code PrivilegedAction} with privileges
   * enabled and restricted by the specified
   * {@code AccessControlContext} and with a privilege scope limited
   * by specified {@code Permission} arguments.
   *
   * The action is performed with the intersection of the permissions
   * possessed by the caller's protection domain, and those possessed
   * by the domains represented by the specified
   * {@code AccessControlContext}.
   * <p>
   * If the action's {@code run} method throws an (unchecked) exception,
   * it will propagate through this method.
   *
   * <p> This method preserves the current AccessControlContext's
   * DomainCombiner (which may be null) while the action is performed.
   * <p>
   * If a security manager is installed and the specified
   * {@code AccessControlContext} was not created by system code and the
   * caller's {@code ProtectionDomain} has not been granted the
   * {@literal "createAccessControlContext"}
   * {@link java.security.SecurityPermission}, then the action is performed
   * with no permissions.
   *
   * @param <T> the type of the value returned by the PrivilegedAction's {@code run} method.
   * @param action the action to be performed.
   * @param context an <i>access control context</i> representing the restriction to be applied to
   * the caller's domain's privileges before performing the specified action.  If the context is
   * {@code null}, then no additional restriction is applied.
   * @param perms the {@code Permission} arguments which limit the scope of the caller's privileges.
   * The number of arguments is variable.
   * @return the value returned by the action's {@code run} method.
   * @throws NullPointerException if action or perms or any element of perms is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedExceptionAction, AccessControlContext)
   * @see java.security.DomainCombiner
   * @since 1.8
   */
  @CallerSensitive
  public static <T> T doPrivilegedWithCombiner(PrivilegedAction<T> action,
      AccessControlContext context, Permission... perms) {

    AccessControlContext parent = getContext();
    DomainCombiner dc = parent.getCombiner();
    if (dc == null && context != null) {
      dc = context.getCombiner();
    }
    if (perms == null) {
      throw new NullPointerException("null permissions parameter");
    }
    Class<?> caller = Reflection.getCallerClass();
    return AccessController.doPrivileged(action, createWrapper(dc, caller,
        parent, context, perms));
  }

  /**
   * Performs the specified {@code PrivilegedExceptionAction} with
   * privileges enabled.  The action is performed with <i>all</i> of the
   * permissions possessed by the caller's protection domain.
   *
   * <p> If the action's {@code run} method throws an <i>unchecked</i>
   * exception, it will propagate through this method.
   *
   * <p> Note that any DomainCombiner associated with the current
   * AccessControlContext will be ignored while the action is performed.
   *
   * @param <T> the type of the value returned by the PrivilegedExceptionAction's {@code run}
   * method.
   * @param action the action to be performed
   * @return the value returned by the action's {@code run} method
   * @throws PrivilegedActionException if the specified action's {@code run} method threw a
   * <i>checked</i> exception
   * @throws NullPointerException if the action is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedExceptionAction, AccessControlContext)
   * @see #doPrivilegedWithCombiner(PrivilegedExceptionAction)
   * @see java.security.DomainCombiner
   */
  @CallerSensitive
  public static native <T> T
  doPrivileged(PrivilegedExceptionAction<T> action)
      throws PrivilegedActionException;


  /**
   * Performs the specified {@code PrivilegedExceptionAction} with
   * privileges enabled.  The action is performed with <i>all</i> of the
   * permissions possessed by the caller's protection domain.
   *
   * <p> If the action's {@code run} method throws an <i>unchecked</i>
   * exception, it will propagate through this method.
   *
   * <p> This method preserves the current AccessControlContext's
   * DomainCombiner (which may be null) while the action is performed.
   *
   * @param <T> the type of the value returned by the PrivilegedExceptionAction's {@code run}
   * method.
   * @param action the action to be performed.
   * @return the value returned by the action's {@code run} method
   * @throws PrivilegedActionException if the specified action's {@code run} method threw a
   * <i>checked</i> exception
   * @throws NullPointerException if the action is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedExceptionAction, AccessControlContext)
   * @see java.security.DomainCombiner
   * @since 1.6
   */
  @CallerSensitive
  public static <T> T doPrivilegedWithCombiner(PrivilegedExceptionAction<T> action)
      throws PrivilegedActionException {
    AccessControlContext acc = getStackAccessControlContext();
    if (acc == null) {
      return AccessController.doPrivileged(action);
    }
    DomainCombiner dc = acc.getAssignedCombiner();
    return AccessController.doPrivileged(action,
        preserveCombiner(dc, Reflection.getCallerClass()));
  }

  /**
   * preserve the combiner across the doPrivileged call
   */
  private static AccessControlContext preserveCombiner(DomainCombiner combiner,
      Class<?> caller) {
    return createWrapper(combiner, caller, null, null, null);
  }

  /**
   * Create a wrapper to contain the limited privilege scope data.
   */
  private static AccessControlContext
  createWrapper(DomainCombiner combiner, Class<?> caller,
      AccessControlContext parent, AccessControlContext context,
      Permission[] perms) {
    ProtectionDomain callerPD = getCallerPD(caller);
    // check if caller is authorized to create context
    if (context != null && !context.isAuthorized() &&
        System.getSecurityManager() != null &&
        !callerPD.impliesCreateAccessControlContext()) {
      ProtectionDomain nullPD = new ProtectionDomain(null, null);
      return new AccessControlContext(new ProtectionDomain[]{nullPD});
    } else {
      return new AccessControlContext(callerPD, combiner, parent,
          context, perms);
    }
  }

  private static ProtectionDomain getCallerPD(final Class<?> caller) {
    ProtectionDomain callerPd = doPrivileged
        (new PrivilegedAction<ProtectionDomain>() {
          public ProtectionDomain run() {
            return caller.getProtectionDomain();
          }
        });

    return callerPd;
  }

  /**
   * Performs the specified {@code PrivilegedExceptionAction} with
   * privileges enabled and restricted by the specified
   * {@code AccessControlContext}.  The action is performed with the
   * intersection of the permissions possessed by the caller's
   * protection domain, and those possessed by the domains represented by the
   * specified {@code AccessControlContext}.
   * <p>
   * If the action's {@code run} method throws an <i>unchecked</i>
   * exception, it will propagate through this method.
   * <p>
   * If a security manager is installed and the specified
   * {@code AccessControlContext} was not created by system code and the
   * caller's {@code ProtectionDomain} has not been granted the
   * {@literal "createAccessControlContext"}
   * {@link java.security.SecurityPermission}, then the action is performed
   * with no permissions.
   *
   * @param <T> the type of the value returned by the PrivilegedExceptionAction's {@code run}
   * method.
   * @param action the action to be performed
   * @param context an <i>access control context</i> representing the restriction to be applied to
   * the caller's domain's privileges before performing the specified action.  If the context is
   * {@code null}, then no additional restriction is applied.
   * @return the value returned by the action's {@code run} method
   * @throws PrivilegedActionException if the specified action's {@code run} method threw a
   * <i>checked</i> exception
   * @throws NullPointerException if the action is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedAction, AccessControlContext)
   */
  @CallerSensitive
  public static native <T> T
  doPrivileged(PrivilegedExceptionAction<T> action,
      AccessControlContext context)
      throws PrivilegedActionException;


  /**
   * Performs the specified {@code PrivilegedExceptionAction} with
   * privileges enabled and restricted by the specified
   * {@code AccessControlContext} and with a privilege scope limited by
   * specified {@code Permission} arguments.
   *
   * The action is performed with the intersection of the permissions
   * possessed by the caller's protection domain, and those possessed
   * by the domains represented by the specified
   * {@code AccessControlContext}.
   * <p>
   * If the action's {@code run} method throws an (unchecked) exception,
   * it will propagate through this method.
   * <p>
   * If a security manager is installed and the specified
   * {@code AccessControlContext} was not created by system code and the
   * caller's {@code ProtectionDomain} has not been granted the
   * {@literal "createAccessControlContext"}
   * {@link java.security.SecurityPermission}, then the action is performed
   * with no permissions.
   *
   * @param <T> the type of the value returned by the PrivilegedExceptionAction's {@code run}
   * method.
   * @param action the action to be performed.
   * @param context an <i>access control context</i> representing the restriction to be applied to
   * the caller's domain's privileges before performing the specified action.  If the context is
   * {@code null}, then no additional restriction is applied.
   * @param perms the {@code Permission} arguments which limit the scope of the caller's privileges.
   * The number of arguments is variable.
   * @return the value returned by the action's {@code run} method.
   * @throws PrivilegedActionException if the specified action's {@code run} method threw a
   * <i>checked</i> exception
   * @throws NullPointerException if action or perms or any element of perms is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedAction, AccessControlContext)
   * @since 1.8
   */
  @CallerSensitive
  public static <T> T doPrivileged(PrivilegedExceptionAction<T> action,
      AccessControlContext context, Permission... perms)
      throws PrivilegedActionException {
    AccessControlContext parent = getContext();
    if (perms == null) {
      throw new NullPointerException("null permissions parameter");
    }
    Class<?> caller = Reflection.getCallerClass();
    return AccessController
        .doPrivileged(action, createWrapper(null, caller, parent, context, perms));
  }


  /**
   * Performs the specified {@code PrivilegedExceptionAction} with
   * privileges enabled and restricted by the specified
   * {@code AccessControlContext} and with a privilege scope limited by
   * specified {@code Permission} arguments.
   *
   * The action is performed with the intersection of the permissions
   * possessed by the caller's protection domain, and those possessed
   * by the domains represented by the specified
   * {@code AccessControlContext}.
   * <p>
   * If the action's {@code run} method throws an (unchecked) exception,
   * it will propagate through this method.
   *
   * <p> This method preserves the current AccessControlContext's
   * DomainCombiner (which may be null) while the action is performed.
   * <p>
   * If a security manager is installed and the specified
   * {@code AccessControlContext} was not created by system code and the
   * caller's {@code ProtectionDomain} has not been granted the
   * {@literal "createAccessControlContext"}
   * {@link java.security.SecurityPermission}, then the action is performed
   * with no permissions.
   *
   * @param <T> the type of the value returned by the PrivilegedExceptionAction's {@code run}
   * method.
   * @param action the action to be performed.
   * @param context an <i>access control context</i> representing the restriction to be applied to
   * the caller's domain's privileges before performing the specified action.  If the context is
   * {@code null}, then no additional restriction is applied.
   * @param perms the {@code Permission} arguments which limit the scope of the caller's privileges.
   * The number of arguments is variable.
   * @return the value returned by the action's {@code run} method.
   * @throws PrivilegedActionException if the specified action's {@code run} method threw a
   * <i>checked</i> exception
   * @throws NullPointerException if action or perms or any element of perms is {@code null}
   * @see #doPrivileged(PrivilegedAction)
   * @see #doPrivileged(PrivilegedAction, AccessControlContext)
   * @see java.security.DomainCombiner
   * @since 1.8
   */
  @CallerSensitive
  public static <T> T doPrivilegedWithCombiner(PrivilegedExceptionAction<T> action,
      AccessControlContext context,
      Permission... perms)
      throws PrivilegedActionException {
    AccessControlContext parent = getContext();
    DomainCombiner dc = parent.getCombiner();
    if (dc == null && context != null) {
      dc = context.getCombiner();
    }
    if (perms == null) {
      throw new NullPointerException("null permissions parameter");
    }
    Class<?> caller = Reflection.getCallerClass();
    return AccessController.doPrivileged(action, createWrapper(dc, caller,
        parent, context, perms));
  }

  /**
   * Returns the AccessControl context. i.e., it gets
   * the protection domains of all the callers on the stack,
   * starting at the first class with a non-null
   * ProtectionDomain.
   *
   * @return the access control context based on the current stack or null if there was only
   * privileged system code.
   */

  private static native AccessControlContext getStackAccessControlContext();


  /**
   * Returns the "inherited" AccessControl context. This is the context
   * that existed when the thread was created. Package private so
   * AccessControlContext can use it.
   */

  static native AccessControlContext getInheritedAccessControlContext();

  /**
   * This method takes a "snapshot" of the current calling context, which
   * includes the current Thread's inherited AccessControlContext and any
   * limited privilege scope, and places it in an AccessControlContext object.
   * This context may then be checked at a later point, possibly in another thread.
   *
   * @return the AccessControlContext based on the current context.
   * @see AccessControlContext
   */

  public static AccessControlContext getContext() {
    AccessControlContext acc = getStackAccessControlContext();
    if (acc == null) {
      // all we had was privileged system code. We don't want
      // to return null though, so we construct a real ACC.
      return new AccessControlContext(null, true);
    } else {
      return acc.optimize();
    }
  }

  /**
   * Determines whether the access request indicated by the
   * specified permission should be allowed or denied, based on
   * the current AccessControlContext and security policy.
   * This method quietly returns if the access request
   * is permitted, or throws an AccessControlException otherwise. The
   * getPermission method of the AccessControlException returns the
   * {@code perm} Permission object instance.
   *
   * @param perm the requested permission.
   * @throws AccessControlException if the specified permission is not permitted, based on the
   * current security policy.
   * @throws NullPointerException if the specified permission is {@code null} and is checked based
   * on the security policy currently in effect.
   */

  public static void checkPermission(Permission perm)
      throws AccessControlException {
    //System.err.println("checkPermission "+perm);
    //Thread.currentThread().dumpStack();

    if (perm == null) {
      throw new NullPointerException("permission can't be null");
    }

    AccessControlContext stack = getStackAccessControlContext();
    // if context is null, we had privileged system code on the stack.
    if (stack == null) {
      Debug debug = AccessControlContext.getDebug();
      boolean dumpDebug = false;
      if (debug != null) {
        dumpDebug = !Debug.isOn("codebase=");
        dumpDebug &= !Debug.isOn("permission=") ||
            Debug.isOn("permission=" + perm.getClass().getCanonicalName());
      }

      if (dumpDebug && Debug.isOn("stack")) {
        Thread.dumpStack();
      }

      if (dumpDebug && Debug.isOn("domain")) {
        debug.println("domain (context is null)");
      }

      if (dumpDebug) {
        debug.println("access allowed " + perm);
      }
      return;
    }

    AccessControlContext acc = stack.optimize();
    acc.checkPermission(perm);
  }
}
